function mascaraTexto(evento, mascara){  
     
   var campo, valor, i, tam, caracter;  
     
   if (document.all) // Internet Explorer  
      campo = evento.srcElement;  
   else // Nestcape, Mozzila  
       campo= evento.target;  
         
   valor = campo.value;  
   tam = valor.length;  
     
   for(i=0;i<mascara.length;i++){  
      caracter = mascara.charAt(i);  
      if(caracter!="9")   
         if(i<tam & caracter!=valor.charAt(i))  
            campo.value = valor.substring(0,i) + caracter + valor.substring(i,tam);  
                 
   }  
  
}